The present invention relates to a mobile communication terminal apparatus and a received signal strength detecting method in a mobile communication system such as a cellular phone, a car phone, and the like.
A handover, which is performed in a mobile communication system using a code division multiple access (CDMA), is implemented when a mobile station detects received signal strengths from peripheral cells and reports the detected received signal strengths to a control station via a base station in communication.
Namely, the mobile station despreads pilot signals received from a plurality of base stations positioned at the peripheral cells, and detects received signal strengths of the respective pilot signals. The control station performs comparison among the received signal strengths of the respective pilot signals of the peripheral cells reported to the control station via the base station in communication by the mobile station. The control station selects a cell providing the highest received signal strength of pilot signals as a handover destination cell, whereby the handover is carried out. The method of the handover in the mobile communication system using CDMA is described in, for example, U.S. Pat. No. 5,267,261.
An explanation will be next given of a received signal strength detecting method that is used by a conventional radio receiving apparatus in the mobile station. FIG. 1 is a block diagram showing a configuration of the conventional radio receiving apparatus.
The respective pilot signals of mobile station peripheral cells received by an antenna 2 of a radio receiving apparatus 1 are subjected to predetermined radio processing and demodulation processing by a receiving section 3. Then, the strength extracting section 4 despreads these pilot signals based on a phase and a spreading code, which are indicated by a control section 5, so as to extract received signal strengths. The received signal strengths of the respective pilot signals are stored in a strength table 6. Then, the strength table 6 calculates average values of the received signal strengths at timing to be described later, and the calculated values are outputted to the other apparatus (not shown) in the mobile station from an output terminal 7. The average values of the received signal strengths of the respective pilot signals are subjected to predetermined processing by the other apparatus (not shown) of the mobile station. Thereafter, the mobile station reports the resultant to the control station via the base station currently in communication.
Next, the following will explain an operation in which the radio receiving apparatus 1 detects received signal strengths of the respective pilot signals of the mobile station peripheral cells. FIG. 2 is a view illustrating that the radio receiving apparatus 1 detects received signal strengths of the respective pilot signals of the mobile station peripheral cells. In FIG. 2, it is assumed that the pilot signals received from peripheral cells A, B, C, and D are pilot signals A, B, C, and D, respectively, the received signal strengths of the respective pilot signals are illustrated under the respective pilot signals, and that the received signal strength increases as the number becomes larger.
A strength extracting section 4 extracts the received signal strengths of the respective pilot signals repeatedly by the same times, and outputs the extracted received signal strengths to the strength table 6. At the time when the received signal strengths of the pilot signals from the same cell are extracted a predetermined number of times, the strength table 6 calculates the average values of the received signal strengths of the respective signals, detects the average values as received signal strengths of the respective pilot signals, and outputs them. The number of extraction times to be predetermined is set to one, which is enough to calculate the average values of the respective pilot signals.
Assuming that the number of extraction times to be predetermined is five. As illustrated in FIG. 2, at timing 17 when the strength extracting section 4 extracts the received signal strength of the pilot signal A five times, the strength table 6 calculates average values of the received signal strengths of the respective pilot signals A, B, C, and D. As a result, the strength table 6 detects the received signal strengths of the respective pilot signals A, B, C, and D as 3, 4, 2, 1, respectively, and outputs them.
The mobile station reports the calculated average values of the received signal strengths of the respective pilot signals to the control station via the base station in communication. Then, the control station selects a base station, which is positioned at the cell B having the pilot signal whose received signal strength is the highest, as a handover destination base station, thereby carrying out the handover.
In the conventional radio receiving apparatus, however, the number of received signal strength extraction times is the same regardless of the received signal strengths of the respective pilot signals A, B, C, and D, and this requires much time to reach a received signal strength detection timing of each pilot signal, and causes a problem in which the selection of the handover destination cell B at the control station is delayed. The more the number of mobile station peripheral cells where the received signal strengths should be detected, the more time is required till received signal strength detection timing of the respective pilot signals is reached, so that the selection of the handover destination cell B at the control station is more delayed.
The delay of the selection of the handover destination cell B causes the handover to the base station, which is positioned at the cell B having the pilot signal whose received signal strength is the highest, to be delayed at the mobile station. This results in the increase of time required for communication between the mobile station and the base station, which is currently in communication and which is positioned at the cell from which the received signal strength is low, so that communication quality is deteriorated.
It is an object of the present invention to provide a mobile communication terminal apparatus and a received signal strength detecting method that carry out a handover speedily and prevent a communication quality from deteriorating by reducing a time required to reach a received signal strength detection timing of each pilot signal.
The inventor of the present invention pays attention to the fact in which the reason why much time is required to reach received signal strength detection timing of the respective pilot signals lies in the point that the number of received signal strength extraction times is the same regardless of the received signal strengths of the respective pilot signals. Then, the inventor of the present invention found out the point in which time required to reach received signal strength detection timing is reduced by increasing the number of received signal strength extraction times with respect to the pilot signals having received signal strengths exceeding the predetermined threshold, and achieved the present invention.
In order to attain the above object, according to the present invention, a cell where a base station, which has transmitted pilot signals having received signal strengths exceeding a predetermined threshold, is positioned is used as a candidate of a handover destination cell. Then, time required to reach received signal strength detection timing is reduced by increasing the number of received signal strength extraction times with respect to the pilot signals having received signal strengths exceeding the predetermined threshold.